Clamps for releasably holding materials during a manufacture process or the releasable locking of tooling such as dies, fixtures or molding plates are known in the art. Such self-locking clamping devices are illustrated in U.S. Pat. Nos. 4,721,293, RE 32,704 and 5,197,720. In U.S. Pat. No. 4,721,293 and RE 32,704, non-positive locking clamping devices are designed to use a fluid such as air or water to move a piston to clamp a workpiece. In U.S. Pat. No. 5,197,720, a non-positive clamping device is shown which incorporates the use of a super elastic alloy. The '720 patent incorrectly uses the term "shape memory alloy" to identify the super elastic alloy. As discussed below, shape memory alloys are a special class of alloys that have shape memory characteristics when heated, whereas super elastic alloys exhibit shape memory characteristics when physical stresses are applied to the alloys. The super elastic alloy in the '720 patent is designed to be mechanically moved by the use of a fluid to thereby lock a workpiece in position. When the workpiece is to be released, the fluid pressure is reduced thereby allowing the super elastic alloy to revert to its original shape and position thereby unclamping the workpiece.
Non-positive clamping tools can provide adequate clamping when properly operated. However, if a failure occurs which would result in a loss of fluid pressure, the clamping forces dissipate thereby allowing the workpiece to be inadvertently released from the clamp. Furthermore, the non-positive clamping device is typically complex in design and requires sophisticated pumps, seals and overall design to operate. The inherent complexity of this design subjects the design to an increased possibility of failure, i.e. pump, valve fitting or line failures. Positive clamping devices overcome the problems associated with non-positive clamping devices; however, such positive clamping devices have typically required complex designs resulting in a clamp that is difficult to repair when it fails.
In view of the prior art of clamping devices, there is a demand for a clamping device which is easy to manufacture, simple in design, reliable in operation and can clamp and unclamp a wide variety of workpieces.